Full build thermal modeling of the additive manufacturing process Tom Stockman, Judy Schneider University of Alabama in Huntsville Bryant Walker Keystone Synergistic Enterprises, Inc. Approved for public release; distribution is unlimited

Objective Develop a model to capture full build cycle thermal history during free form additive manufacturing Model should be capable of integration into a production environment Center for Advanced Vehicular Systems, Mississippi State University

Additive manufacturing is governed by heat transfer Effective thermal management can minimize several concerns: Slumping Residual stress Microstructural management Void distribution Geometrical tolerancing High heat input build Low heat input build

Modeling the full build is imperative Thermal history of the build Substrate temperature distribution Melt pool Modeling the full build can also offer insight into microstructural evolution

Requirements for incorporation into a production environment Quick turnaround Predictive capability Integration with CNC software (G code) Compatibility with current processes: Powder bed processes: laser, electron beam Freeform processes Blown powder: laser, electron beam Wire fed: laser, electron beam, pulsed-arc

Capabilities with pulsed arc, wire fed, free form additive manufacturing process Avoids safety issues related to fine powders Not confined to typical dimensions of a powder bed system Applicable to deposition of multiple materials Selection of heat source controls deposition rates

Free form wire fed with-pulsed arc heat source provides a production platform for model verification Thermal measurement sensor on robotic head Multi-axis robotic platform Arc length control Z axis build height sensor

Thermocouples and infrared (IR) thermal measurements provide thermal data to verify model Materials used in builds: Inconel 718, Inconel 625, Invar 36 steel, Maraging steel, 4340 steel

Complex shapes yield a complex thermal profile 14.5 cm diameter (bottom) 10 cm long x 10 cm high

Most thermal modeling of AM process focus on localized temperatures Models start from melt pool temperature, shape, and size Thermal history tied to residual stress Many models based on commercial software (ABAQUS) Time intensive Currently not targeting full build Chin, Beuth, Amon, J. Mfgt Sci. & Engr., Vol. 123, 2000. Cheng, Price, Lydon, Cooper, Chou, J. Mfgt Sci & Engr, Vol 136, 2014.

A customized model offers the ability to rapidly import varying geometries and model the full build Custom built in free open source programming language (Python 2.7) Non-linear, transient, mass added numerical model “G” code compatible for ease of use in production environment Maintains thermal history of entire build Temperature predictions can be used to program robotic controls for desired build temp

Boundary conditions are convection and radiation, applied differently depending on direction Free convection in 3 directions Quasi-static gaseous environment Cartesian mesh Black body radiation

Temperature evolution governed by heat flux Heat flux induced by mass addition at melt temperature Flux verified by comparison to Rosenthal equations and by estimates from The Welding Institute Incropera, F., 2007, Fundamentals of Heat and Mass Transfer 6th Edition The Welding Insitute, www.twi-global.com

Each step of the deposition follows a time discretized process flow

Example of 2D model output 2D model migrated to 3D

Summary Instructions built with times relative to the start of each pass allowing top layer of build to cool to given interpass temperature before next layer begins Modeling allows for optimization of interpass temperature Hold times used for temperature control based on geometrical and thermal profile Model is being developed to allow future integration with other tools to optimize process parameters and predict microstructural evolution and residual stresses

Acknowledgements Keystone Synergistic Enterprises, Inc., "Extension of Physics Based Laser MDDM Process Mapping,” Internal IR&D. NASA STTR Phase I with Keystone Synergistic Enterprises, Inc., “Advancing Metal Direct Digital Manufacturing (MDDM) Processes for Reduced Cost Fabrication of Bi-Metallic Cooled Rocket Engines.”